Rauner, a multi-millionaire who served one term, issued his concession less than an hour after polls closed, recounting his phone conversation with Pritzker moments prior: “I said, Godspeed. I hope and pray you serve Illinois well.”

Pritzker, whose net worth is estimated at $3.5 billion, spent some $171.5 million of his own money on the campaign, the Chicago Tribune reported, paying for an onslaught of TV and online ads attacking Rauner and building his own name recognition.

The billionaire Pritzker's family name is ubiquitous around Chicago, where it adorns the bandshell in Millennium Park, Northwestern University's law school, the University of Chicago's medical school, a military museum, and a children's zoo.

Pritzker himself was born in California, where his father had moved to help run the fledgling Hyatt hotel chain.

He became involved in Democratic politics in his 20s, campaigning for Senator Ted Kennedy in his 1980 presidential primary bid against Jimmy Carter, and going on to work on Capitol Hill for Democratic Senators Terry Sanford of North Carolina and Alan Dixon of Illinois.

Pritzker returned to his family's longtime bastion of Chicago in the 1990s, attending law school at Northwestern before going on to run a failed campaign for U.S. House in 1998.

Despite Barack Obama's Illinois ties as a US Senator, Pritzker backed Hillary Clinton in the 2008 presidential primary, serving as a national co-chair of her campaign.

After Obama's primary victory, Pritzker helped unite the two camps for the general - and Obama repaid the favor by campaigning for the billionaire in Chicago ahead of Tuesday's election.

Last year, Pritzker became embroiled in one of Illinois' most embarrassing gubernatorial scandals after FBI wiretaps of him emerged having a 2008 conversation with then-governor Rod Blagojevich.

Blagojevich, who was later sentenced to 14 years in prison for soliciting bribes for political appointments, and Pritzker discuss the idea of the billionaire's appointment to the office of state treasurer.

'That's the one I would want,' Pritzker tells Blagojevich on the tapes obtained by the Tribune. However, Pritzker was never charged with illegal conduct in the matter, and insists he did nothing to break the law.

Pritzker also made headlines last month, when he was accused of a 'scheme to defraud' taxpayers by removing toilets from a Chicago mansion and declaring it 'uninhabitable' as part of a property tax reassessment that saved him $330,000.

A review by Cook County Inspector General Patrick Blanchard also found that family members and associates made 'false representations' to the county assessor about the condition of the Chicago mansion.

The allegations were not enough to derail Pritzker's campaign, after incumbent Rauner pumped $50 million of his own money into the race but had no hope of keeping pace with Pritzker's spending.

Rauner was elected in 2014 on a platform of cleaning up corruption in the state capital, but angered his Republican base by signing laws expanding gay, abortion, and immigrant rights, and nearly lost his primary in this election.

When he is sworn into office in January, Pritzker will step into what has been described as the worst job in American politics.

Illinois has teetered on a financial precipice for years, with billions in unfunded pension liabilities and a credit rating near junk status.

Rauner himself called the state a 'banana republic' during a bitter standoff with Democrats in the legislature that left Illinois with $7 billion in unpaid bills.

Pritzker will become the richest governor in U.S. history, and the second richest elected official, after former New York Mayor Michael Bloomberg.